A crude oil residuum or heavy oil which is often referred to as asphaltic fractions in the refining of crude oil is broadly understood to be the residue obtained from crude oil after a nondestructive distillation has removed substantially all of the volatile fractions. Refining temperatures are usually maintained below about 540.degree. C. (1000.degree. F.), and storage temperatures below about 350.degree. C. (660.degree. F.) as the rate of thermal decomposition of petroleum becomes substantial above such temperature. Residua are black, viscous materials and are obtained as a residue from atmospheric or vacuum distillation of a crude oil. They may be liquid at room temperature (generally atmospheric residua) or almost solid (generally vacuum residua) depending upon the crude oil.
The organic chemical composition of residua is complex and may contain ash-forming metallic constituents and sulfur compounds, since metals and sulfur compounds of one type or another are generally present in crude oil. In residua, there are many varieties of sulfur compounds depending on the prevailing conditions during the formation thereof. The presence of the sulfur compounds in the residua gives rise to the generation of a gas having substantial portions of hydrogen sulfide gas.
Residua have found extensive use as a bunker fuel oil, No. 6 fuel oil, fuel oil C, and marine fuel oil. Residua must be transported from the refinery to the points of use, such as a ship or a power generating plant. Unfortunately, during storage or such transport, hydrogen sulfide gases become liberated and give rise to a multitude of environmental problems.
Hydrogen sulfide is a very toxic gas and; thus, for safety purposes, the use of residua requires special handling. The contamination of residua with hydrogen sulfide forming substances thus presents a series of problems as the residua are stored or transported. Providing an effective chemical method for suppressing or inhibiting the liberation of hydrogen sulfide gases from residua is of considerable importance to the petroleum refining industry. Methods heretofore known for suppressing the liberation of hydrogen sulfide gases from residua suffer from the standpoint of effectiveness.
Hydrogen sulfide scavengers for use in other media are known. However, such scavengers are not recognized to have universal application and to be effective in widely differing media. For several reasons, the efficacy of such hydrogen sulfide scavengers is particularly problematic with respect to the media to which the present invention is directed (i.e., crude oil, petroleum residua and fuels). For example, as noted, petroleum residua are very complex and impure, containing a multitude of unknown compounds, providing ample opportunity for side reactions. The same, of course, is true for crude oil from which the residua are derived. Fuels, in particular mid-distillate fuels, such as kerosene and diesel fuels, while more refined, still contain a multitude of compositions. Accordingly, the scavenger must be very selective as well as fast acting. Moreover, the applications to which the media of the present invention are directed, for example, burning in engines, demand many other considerations, such as the ability to avoid the formation of residue.
Thus, for example, while compositions such as neutralizing amines, iron compounds and certain oxidizing compounds such as sodium hydroxide, are useful for suppression of hydrogen sulfide formation in cutting oils, they have been found to be unsuitable for use in the media of concern here. In particular, neutralizing amines have not been found to be thermally stable at temperatures to which such media are subjected. Iron compounds, upon combustion, form ash which is impermissible in applications such as use in turbine engines.
Certain types of oxidizers act by conversion of hydrogen sulfide to elemental sulfur. Because of the high reactivity of elemental sulfur, it tends to reform hydrogen sulfide in the media to which the present invention is directed. In addition, such oxidizers, examples of which include sodium hypochlorite and sodium nitrite, have deleterious affects on fuel and are dangerous to use. Other types of additives, such as sodium hydroxide, act as neutralizers, thereby forming end products, for example, sodium sulfide or sodium hydrogen sulfide, from the hydrogen sulfide. The insolubility and non-volatility of such end products results in the formation of deposits in engines. In addition, sodium is known to cause corrosion at high temperatures, and has been found to react with the acid present in the impure media of concern herein, thus limiting its usefulness as a scavenger. Sodium hydroxide also has been found to have very limited efficacy in scavenging hydrogen sulfide in fuels. Thus, such oxidizers and neutralizers are not suitable for use in the media of the present invention. Various other oxidizers are not suitable for the media of concern herein because they react with a large number of the compounds present in the media.
U.S. Pat. No. 4,778,609 to Koch et al. describes the use of certain hindered monoimines to suppress the generation of hydrogen sulfide emissions in lubricating oil caused by the introduction of certain organic sulfides. However, such hindered monoimines are not as economical or commercially available as desired, nor is there any indication that such compositions (which are used by Koch et al. to treat relatively pure media in which side reactions are not of concern) would be sufficiently selective, fast-acting, and free of deleterious side effects to be useful in the difficult conditions associated with the complex media of concern in the present invention. In fact, especially in view of the statement at lines 46-55 of Column 2 of the Koch et al. patent that the scavengers disclosed therein successfully suppress hydrogen sulfide generation for only certain sulfur compounds, the Koch et al. patent contains no suggestion that the scavengers, which are used therein for certain organo-sulfur compounds added to the lubricating oil by Koch et al. would have any effectiveness at all with respect to the sulfur compound inherent in the media of concern herein.
Cole et al. U.S. Pat. No. 3,053,645 describes the use of certain condensation products of aldehyde and certain fatty diamines (having one primary amino group) in distillate fuel oils as stabilizers. These products are directed not to hydrogen sulfide scavenging, but to antioxidation. Stability and prevention of oxidation is of concern in distillate fuels, but not in crude oil or petroleum residua. In other words, whereas oxidation is not recognized as a problem in such unrefined media and treating such media would merely duplicate efforts because another treatment after refining would be required. Treatment of refined media is required to maintain product quality to avoid the necessity for reprocessing to render them suitable for use. Thus, the Cole et al. patent contains no teaching or suggestion of hydrogen sulfide scavenging in any media, or of treating crude oil or petroleum residua for any purpose.
Andress, Jr. et al. U.S. Pat. No. 3,449,424 is directed to certain acidic salicyladimines to inhibit corrosion, but contains no teaching or suggestion of any technique for inhibiting hydrogen sulfide generation. Thus, the Andress, Jr. et al. patent discloses the use of such compositions in media in which corrosion can be a problem (e.g., hydrocarbon fuels, lubricating oils and greases), as opposed to such generally aqueous-free media as crude oil or petroleum residua. Moreover, the acidic nature of such compositions renders them inapplicable for the media of concern in the present invention, where the acid reacts with amines and other various components present in the medium. Typically, compositions which contain acidic groups (e.g., phenolic or carboxylic groups) are employed as in Andress, Jr. et al. as corrosion inhibitors due to their ability to form a complex with iron and thereby to form a protective layer over iron surfaces. They do not act as scavengers.
Accordingly, there is still a need for economical, easily accessible, hydrogen sulfide scavengers that are sufficiently selective, fast-acting, non-residue producing and stable for use in crude oil, petroleum residua and fuels.